What is The Main Causes of Subsidence?

Subsidence is a serious issue that can affect homes and buildings. It happens when the ground beneath a structure sinks or shifts, causing damage to the foundation and walls.

Subsidence can be caused by soil types, nearby trees, water leaks, and other factors. We have found that most homes repaired by Home Subsidence Repairs are caused by water leaks that gradually got worse over time.

You might spot signs of subsidence in your home. These can include cracks in walls wider than 3mm, especially near doors and windows, as per HOA.org.uk. Doors and windows that stick or don’t close properly can also point to subsidence issues.

The causes of subsidence vary. Shallow foundations in older homes can be at risk. Changes in soil moisture from drought or leaks can lead to ground movement. Tree roots can dry out the soil, and mining or quarrying can weaken the ground.

1. Geological Causes of Subsidence

Subsidence can occur due to various natural processes and human activities that affect the ground beneath us. The geological factors play a key role in how and why the land sinks or settles over time.

Soil Properties and Composition

Certain soil types are more prone to subsidence than others. Clay soils, for example, can shrink when they dry out and swell when wet. This constant change can lead to ground movement.

Sandy soils may be unstable and prone to erosion by water or wind. When water flows through sandy soil, it can wash away particles, creating voids underground.

Peat soils are particularly susceptible to subsidence. As peat dries out, it compresses and loses volume, causing the ground above to sink.

Natural Erosion Processes

Water is a major factor in natural subsidence. Rivers and streams can erode their banks and beds over time, weakening the surrounding ground.

Coastal erosion by waves and tides can undermine cliffs and shorelines, leading to collapse and subsidence of the land above.

In karst terrains, water dissolves limestone, creating underground cavities. If these cavities grow too large, the roof may collapse, causing sinkholes at the surface.

Earthquakes can trigger sudden subsidence by shaking and destabilising the ground. They may also cause liquefaction of sandy soils, leading to rapid sinking.

Historical Mining Activities

Past mining operations have left many areas vulnerable to subsidence. When minerals are extracted, the ground above can become unstable.

Underground coal mines are a common cause of subsidence. As old mine shafts and tunnels collapse, the ground above settles.

Quarrying for stone or gravel can also lead to instability. The removal of large amounts of material can change the local stress patterns in the ground.

Salt mining has caused significant subsidence in some areas. As salt is dissolved and extracted, cavities form underground, potentially leading to collapse.

2. Human-Induced Factors

Human activities play a big role in causing land subsidence. These actions change the ground beneath our feet in ways that can make it sink.

Urban Development and Land Use

Cities grow fast, putting pressure on the land. As buildings go up, their weight pushes down on the soil. This can squeeze out water and air, making the ground compact.

In some places, developers drain wetlands to build on. This dries out the soil, causing it to shrink. When soil loses water, it can collapse under the weight above it.

Land use changes also affect subsidence. Cutting down trees and paving over green spaces stops water from soaking into the ground. This can dry out soil layers underneath.

Water Extraction

Taking too much water from underground can lead to sinking land. As we pump water out, the spaces it filled collapse. This makes the ground above settle.

Groundwater pumping is a big cause of subsidence in many areas. Farms, factories, and cities all use lots of water. When more is taken out than rain puts back, the land can sink.

In coastal areas, overpumping can let salt water in. This can make the problem worse by weakening soil structure.

Infrastructure Projects

Big building projects can cause the ground to sink. Digging tunnels for roads or trains removes soil and rock. This can make the land above settle.

Mining is another major cause. Taking out coal, oil, or gas leaves empty spaces underground. Over time, these can collapse, causing the surface to sink.

Dams and reservoirs put extra weight on the land. This can squeeze out water from soil and rock below, leading to subsidence around the edges.

Building on soft soils without proper support can cause sinking. As structures settle, they can drag down the surrounding land.

3. Climatic Influences on Subsidence

Climate plays a big role in land subsidence. Changes in weather patterns can affect soil stability and groundwater levels, leading to ground movement.

Drought and Precipitation Patterns

Dry spells can cause the ground to sink. When soil dries out, it shrinks and loses volume. This shrinkage can make the land surface drop.

In areas with clay-rich soils, the effects can be dramatic. Clay soils swell when wet and shrink when dry. This process of swelling and shrinking can damage building foundations.

Heavy rains after drought can worsen subsidence. Dry, cracked soil absorbs water quickly. This rapid uptake can weaken soil structure and cause further sinking.

Climate change is altering rainfall patterns in many regions. More frequent droughts and intense storms may increase subsidence risks in some areas.

Vegetation and Root Systems

Plants play a key role in soil stability. Their roots hold soil particles together and help prevent erosion.

During droughts, plants may die off. This loss of vegetation can lead to soil instability and increased subsidence risk.

Some plants, like trees, can actually cause localised subsidence. Their roots draw large amounts of water from the soil. In clay soils, this can lead to shrinkage and ground movement.

Climate change may alter plant communities in some areas. These shifts could affect soil stability and subsidence patterns over time.

Proper land management can help reduce climate-related subsidence risks. This includes choosing plants suited to local conditions and managing water use wisely.

4. Detection and Monitoring of Subsidence

Spotting and tracking land subsidence requires special tools and methods. These help experts measure changes in ground level over time.

Surveying Techniques

Land surveyors use precise tools to measure ground height changes. They set up benchmarks, which are fixed points with known heights. Then they check these points regularly to see if they’ve moved.

One key tool is the total station. It measures angles and distances to calculate exact positions. GPS systems are also useful. They can track tiny shifts in land position over wide areas.

Levelling is another important method. It uses a level instrument and a staff to measure height differences between points. This can show if an area is sinking.

Tiltmeters are used to detect small changes in ground slope. These are often placed in buildings or on structures to monitor their movement.

Remote Sensing

Satellite technology lets us watch subsidence from space. Interferometric Synthetic Aperture Radar (InSAR) is a powerful tool. It can measure surface changes as small as a few millimetres.

InSAR works by comparing radar images taken at different times. This shows where the ground has moved. It’s great for watching large areas and spotting patterns.

LiDAR (Light Detection and Ranging) is another useful method. It uses lasers to create detailed 3D maps of the ground. By comparing maps from different times, experts can see where subsidence has occurred.

Aerial photos can also help. When taken over time, they can show visible changes in land levels. This is especially useful in urban areas where subsidence might affect buildings.

Deep learning techniques are now being used with these remote sensing methods. They help process the huge amounts of data collected and can even predict future subsidence.

5. Risk Assessment and Management

Evaluating and managing subsidence risks involves careful analysis and strategic planning. Implementing effective measures can help mitigate potential damage and financial losses.

Building Codes and Standards

UK building codes set guidelines for construction in subsidence-prone areas. These aim to reduce risks through proper foundation design and materials. The British Geological Survey offers property subsidence assessment tools to help identify hazards.

Key standards include:

• Minimum foundation depths
• Soil testing requirements
• Drainage specifications

Builders must follow these rules to get planning permission. Regular inspections check compliance. Updates to codes happen as new research emerges on subsidence causes and prevention.

Insurance and Liability

Home insurance often covers subsidence damage, but policies vary. You need to check your coverage details. Some insurers use risk assessment data to set premiums in high-risk areas.

Common policy features:

• Excess amounts for subsidence claims
• Coverage for repairs and stabilisation
• Exclusions for certain types of damage

If you spot signs of subsidence, tell your insurer right away. Quick action can limit damage and keep claim costs down. In some cases, you might need specialist subsidence insurance.

Landlords have extra duties to protect tenants from subsidence risks. This includes maintaining buildings and fixing issues promptly.

6. Remediation and Prevention Measures

Addressing subsidence requires a multi-faceted approach. Several techniques can help stabilise affected structures and prevent future issues.

Foundation Strengthening and Underpinning

Underpinning is a key method for reinforcing foundations affected by subsidence. This process involves extending the foundation depth or width to distribute the building’s weight over a larger area.

Common underpinning techniques include:

• Mass concrete underpinning
• Beam and base underpinning
• Mini-piled underpinning

Each method suits different soil types and building structures. Mass concrete works well for shallow foundations, while mini-piles are ideal for deeper, more challenging ground conditions.

Foundation repair can also involve resin injection. This technique fills voids in the soil beneath the foundation, providing additional support. It’s an alternative worth considering due to being less disruptive than traditional underpinning and often quicker to complete.

Soil Stabilisation Techniques

Soil stabilisation aims to improve ground conditions and prevent further subsidence. Grouting is a popular method, involving the injection of cement or chemical mixtures into the soil.

Other soil stabilisation methods include:

• Compaction grouting
• Jet grouting
• Deep soil mixing

These techniques can strengthen weak soils, fill voids, and create a more stable foundation for buildings. The choice of method depends on soil type, depth of treatment needed, and site accessibility.

For clay soils prone to shrinkage, tree root barriers can help. These physical barriers prevent roots from drawing moisture from the soil near foundations, reducing the risk of subsidence.

Sustainable Water Management

Controlling groundwater is crucial in subsidence prevention, especially in areas with high water tables or mining activity.

Proper drainage systems can help manage water levels and reduce soil saturation.

Key water management strategies include:

• Installing French drains
• Improving surface water drainage
• Repairing leaky pipes and sewers

In areas prone to drought, maintaining consistent soil moisture can prevent clay shrinkage.

This might involve controlled irrigation during dry periods or installing moisture sensors to monitor soil conditions.

For regions with excessive groundwater extraction, implementing water conservation measures and exploring alternative water sources can help maintain groundwater levels and reduce subsidence risks.